This invention relates to an improved process for preparing metal phthalocyanine pigments, particularly copper phthalocyanines, in which certain dialkyl phthalate esters are used as the reaction solvent.
The preparation of metal phthalocyanine pigments is known. In a typical preparation, a phthalocyanine-forming material based on phthalic acid, such as phthalic anhydride or a derivative thereof, is heated with a metal donor, a nitrogen donor, and an optional catalyst, preferably in an organic solvent. Many different solvents have been described for use in the preferred solvent processes. For example, U.S. Pat. No. 2,549,842 discloses aromatic hydrocarbons and halogenated derivatives thereof, as well as mixtures of such solvents with aliphatic diluents such as kerosene or chlorinated kerosene derivatives; U.S. Pat. No. 2,824,107 discloses inert organic solvents such as trichlorobenzene, chlorobenzene, naphthalene and chlorinated derivatives thereof, quinoline, benzophenone, and nitrobenzene; U.S. Pat. No. 2,910,482 discloses nitrobenzene, o-nitrotoluene, trichlorobenzene, o-dichlorobenzene, and kerosene; U.S. Pat. No. 3,057,872 discloses trichlorobenzene and kerosene; U.S. Pat. No. 3,300,512 discloses inert high boiling organic solvents such as nitrobenzene, trichlorobenzene, o-dichlorobenzene, kerosene, and naphthalene; U.S. Pat. No. 3,615,805 discloses kerosene; U.S. Pat. No. 4,785,091 discloses hydrophilic solvents, such as sulfolane, 2-methylsulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, dimethylsulfoxide, N-methylpyrrolidone, and 1,3-dimethyl-2-imidazolidinone; French Patent 2,664,898 discloses inert chlorinated aromatic solvents, such as dichlorotoluene isomers; Japanese Patent 82/141,453 discloses 2-ethylhexyl benzene; Japanese Patent 82/145,156 discloses dimethylethylbenzene (used under pressure); Japanese Patent 82/149,358 discloses alkylbenzenes, including tert-hexylbenzene; Japanese Patent 82/151,654 discloses inert organic solvents, such as amyl alcohol, dichlorobenzene, trichlorobenzene, nitrobenzene, and chloronaphthalene; Japanese Patent 83/21,452 discloses dichlorotoluene and chloroxylene; Japanese Patent 84/89,361 discloses tert-hexylbenzene; Japanese Patent 91/161,489 discloses tert-amylbenzene; and Chinese Patent 1,037,722 discloses bicyclohexyl.
Certain ester solvents have also been reported. For example, Czech. Patent 202,609 discloses methyl 4-methylbenzoate, optionally in admixture with methyl or ethyl benzoate and Japanese Patent 77/10,326 discloses lower alkyl benzoates.
Each of the previously reported solvents, however, is attended by one or more disadvantages, including toxicity, environmental risk, inadequate availability, high cost, processing difficulties (e.g., because of need for large volumes, high pressures, difficult recovery for recycling or disposal, and the like), and low product yields.
It has now been found that these and other difficulties can be avoided or minimized by the use of a di(C.sub.1 -C.sub.3 alkyl) phthalate as the reaction solvent in the preparation of metal phthalocyanine pigments. For example, dimethyl phthalate, the preferred solvent of the invention, is relatively inexpensive, need not be used in large volumes, and provides excellent yields of pure metal phthalocyanine pigments. Moreover, dimethyl phthalate is not irritating to or absorbed through the skin and is only moderately toxic when ingested. E.g., The Merck Index, 10th edition (Merck & Co., Inc., Rahway, N.J., 1983) at page 474. Although dialkyl phthalates have been used as conditioning solvents for phthalocyanines that have already been formed chemically but not yet converted into pigmentary form (e.g., U.S. Pat. No. 3,041,192 and European Patent Application 58,888), dialkyl phthalates have not previously been disclosed as suitable reaction solvents for the phthalocyanine-forming step.